QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
In a typical QKD system, Alice and Bob are optically coupled by an optical fiber that carries only the quantum signals. This facilitates detecting the quantum signals because there are no externally introduced sources of noise from other optical signals. However, incorporating QKD into existing optical networks requires that the quantum signals share the same optical fiber as “classical” (i.e., non-quantum) optical signals used in standard optical telecommunications. This complicates the QKD process because detecting the quantum signals is hampered by the presence of the classical signals as well as the relatively large amounts of noise they generate in the quantum channel.
One approach to transmitting quantum signals over an optical fiber populated by high-speed traffic in the form of classical signals is to transmit the quantum signals on a substantially different wavelength band than the classical signals. Such an approach is discussed in the article by Chapuran et al., entitled “Compatibility of quantum key distribution with optical networking,” Proc. SPIE Vol. 5815 (2005), which article is incorporated by reference herein. The present state of the prior art teaches that the quantum and classical signals should be transmitted in wavelength bands separated by at least 150 nm. However, it is preferable to transmit both signals at much closer wavelengths. For example, in some cases it is advantageous to transmit both types of signals within the third telecom window (i.e., wavelengths around 1500 nm). The quantum signals benefit from the extended reach gained by operating near the optical fiber's maximum transparency wavelength (1550 nm) in this band. The classical signals also benefit from low attenuation as well as the wider variety of quality optical components available for these wavelengths, such as optical amplifiers.